1. Field of the Invention
The present invention relates to: an inkjet printing apparatus including printing-medium conveying units arranged upstream and downstream of a printing unit which performs printing on a printing medium; and a control method for the apparatus. Specifically, the present invention relates to a technology to prevent deterioration in a printing quality due to a conveying error of a printing medium after the printing medium is released from the conveying unit arranged upstream of the printing unit.
2. Description of the Related Art
A printing apparatus generally includes a conveying unit which conveys a printing medium along a predetermined conveying path, and a printing unit which performs printing on the conveyed printing medium. Moreover, another printing apparatus may include conveying units both upstream and downstream, in a printing-medium conveying direction, of a position where a printing unit performs printing (printing position). Hereinafter, the conveying units are referred to as an upstream conveying unit and a downstream conveying unit. These upstream and downstream conveying units are involved in a printing-medium conveying operation including the supplying of a printing medium to the printing position and the discharging of the printing medium from the printing position. Generally, the upstream and downstream conveying units include a conveying roller and a discharge roller, respectively.
The conveying roller is configured of a metallic roller processed to have fine asperities on the surface thereof, and thereby to generate a large friction. Meanwhile, the discharge roller is configured of a roller made of a material having a large friction coefficient such as a rubber. Each of the conveying roller and the discharge roller is provided with a pinch roller for nipping a printing medium. The pinch roller is resiliently biased by a pressing member such as a spring. Stated differently, the upstream conveying unit is formed of the pair of the conveying roller and the corresponding pinch roller. The downstream conveying unit is formed of the pair of the discharge roller and the corresponding pinch roller.
The roller diameters and the driving systems of the conveying and discharge rollers are set so that the discharge roller can have a peripheral velocity higher than the conveying roller by approximately 0.3% to 1%. Meanwhile, the nipping force by the roller pair in the downstream conveying unit is set to be smaller than that in the upstream conveying unit. Under these settings, both of the conveying units nip and convey a printing medium without slackening thereof, so that the surface of the printing medium to be printed is kept flat. In addition, the printing medium is allowed to slip on the downstream conveying unit. Accordingly, no inappropriate load is generated. This mechanism is effective when being used for a printing unit which performs a non-contact printing on a printing medium, and particularly for an inkjet printing head (hereinafter, also simply referred to as a printing head) which performs printing by ejecting a liquid ink on a printing medium. This is because, when such a printing head is used, the distance between the ejection surface (the surface of the printing head, on which ejection outlets are provided) and the printing surface of a printing medium is strongly desired to be maintained constant in order to maintain the printing quality, and to prevent the ejection surface and the printing medium from coming into contact with each other.
Printing progresses on a printing medium in a state where the printing medium is nipped by both of the upstream and downstream conveying units (first conveying state), and then is about to reach the rear end of the printing medium. At this time, the rear end of the printing medium comes off from the position nipped by the upstream conveying unit. Thus, the printing medium is switched to a state of being nipped only by the downstream conveying unit (second conveying state). Since the downstream conveying unit conveys the printing medium at a relatively high speed, the printing medium is conveyed at the speed higher than that in the first conveying state. As a result, an excessive conveying may occur, causing the deterioration in the image quality on the printing medium such as a white stripe and color drift.
In order to solve this problem, known is a method in which a conveyance amount of the printing medium is corrected before and after the rear end of the printing medium is passed through the upstream conveying unit (for example, Japanese Patent Laid-open No. 8-282027 (1996)).
However, even when the conveyance amount in a printing apparatus with an inkjet printing head is corrected by using the method disclosed in Japanese Patent Laid-open No. 8-282027 (1996), the deterioration in the image quality such as the occurrence of a stripe on the printed image is not always suppressed. To be more specific, the present inventors have discovered that such a uniform correction is not sufficient to suppress the deterioration in the image quality, because the printing media are expanded or contracted by different amounts from each other during printing. In particular, the present inventors have discovered that use of a printing medium subjected to swelling by the application of liquid leads to the significant deterioration in the image quality due to an error in conveying the printing medium to a printing position, especially when a large amount of ink is applied to the printing medium. In addition, one of the factors causing the conveying error is, for example, that different types of the printing media are expanded or contracted by different amounts. The details of this discovery will be described as follows.
In the first conveying state, since the downstream conveying unit allows the printing medium to slip, the conveying speed or conveying distance is determined by the upstream conveying unit. Moreover, ink is not actually applied to a portion of the printing medium between the upstream conveying unit and the printing position. Thus, the printing medium is hardly influenced by the expansion or contraction of itself, and the error in the conveyance amount due to the difference in the expansion/contraction amount is not likely to occur. In contrast, in the second conveying state, ink may be applied to a portion of the printing medium between the printing position and the downstream conveying unit. In this case, a difference in the ink absorbing property and other properties of the printing media leads to variations in the expansion/contraction amount among the printing media. For this reason, even when the conveyance amount is uniformly corrected, the error in the conveyance amount to the printing position is still large, and thereby a stripe occurs in the printed image.